The present invention relates to a load monitoring apparatus for monitoring a load of various machineries, such as machine tools, each incorporating an electric motor as a drive source thereof.
If an abnormal load is generated in a machinery, such as a machine tool or a carrying apparatus, incorporating an electric motor (hereinafter simply called a "motor") as a drive source thereof, continuous rotation of the abnormal motor will cause the motor to be burnt, the tool to be damaged and the machining efficiency to excessively be lowered. Therefore, the load of the machinery must be monitored while it is driving.
Hitherto, the electric-power value of the motor which directly corresponds to the load which is thus increase/decrease has been detected. An operation for adjusting the electric-power value of the motor to satisfy an appropriate absolute-value range has been performed so that the load has been met the appropriate absolute-value range.
When the operation for monitoring the load by detecting the electric-power value is performed to, for example, detect a fact that a cargo is slightly caught in the carrying apparatus in accordance with change in the electric-power value of the motor, the weight of the cargo is usually different in each case. Therefore, when a very light cargo is carried, the method of monitoring the load by detecting the absolute-value range of the electric-power value has a probability that an abnormal load cannot be determined even if an excessively heavy load is applied to the machinery. If the abnormal load is detected, the machinery has already been brought to a dangerous state.
Therefore, the applicant of the present invention has disclosed a load monitoring apparatus in Japanese Patent Laid-Open No. 7-274383. The load monitoring apparatus has a structure that the electric-power value supplied to the motor is monitored in accordance with upper and lower limits for defining a predetermined absolute-value range for the electric-power value. Moreover, electric-power values of the motor extracted at a predetermined sampling cycle are previously stored. The electric-power value is monitored in accordance with upper and lower limits for defining a relative-value range in which the previously extracted electric-power value is employed as a referential electric-power value. If the extracted electric-power value does not satisfy either of the absolute-value range or the relative-value range, an abnormal signal is output. Moreover, electric-power value supply to the motor is cut off. The above-mentioned load monitoring apparatus, for example, is able to monitor relative change in the load according to the change in the cargo weight.
The above-mentioned machinery has a reduction gear provided for the drive system thereof. If foreign matter is introduced into bearings which support a rotational shaft in the reduction gear, a continuous driving of the reduction gear causes the inner surface of the bearings to be scratched. Thus, the load is gradually increased. If the driving is continued, the bearings will be burnt and thus the machinery becomes unoperatable.
If the load is gradually increased, the electric-power value of the motor does not exceed the upper limit of the relative- value range which is updated at each sampling cycle. When the electric-power value exceeds the upper limit an abnormal signal is output. There is apprehension that the bearings have been burnt at this time and thus the machinery becomes unoperatable. Therefore, development of a load monitoring apparatus capable of quickly producing an output of the abnormal signal has been required.
To meet the above-mentioned requirement, it might be considered feasible to arrange the load monitoring apparatus disclosed in Japanese Patent Laid-Open No. 7-274383 such that the sampling cycle is elongated maximally so as to increase the difference from the previous electric-power value which serves as the reference electric-power value.
However, in a reduction gear having a structure that lubricating oil in a large quantity is circulated in the inside unit thereof, the electric-power value is initially rapidly increased by dint of the starting electric-power value after the motor has been started, and then, the electric-power value is rapidly decreased even if the load is in a normal state. The electric-power value is larger than that in a case where the drive is performed in a state in which the viscosity of the lubricating oil in the reduction gear is stable. In this case, the electric-power value is considerably larger than a predetermined upper limit for the absolute-value range. If the drive is continued, the viscosity of the oil is lowered because the temperature of the lubricating oil in the reduction gear is gradually raised. Therefore, the electric-power value is gradually decreased until it is stabilized.
The electric-power value of the machinery having such reduction gear changes more considerably as compared with the above-mentioned case in which the foreign matter is introduced into the bearings. Therefore, if the sampling cycle is elongated maximally as described above, the conventional load monitoring apparatus cannot respond to the rapid change in the load which occurs in a period from start of the motor to a moment at which the transmission efficiency of the reduction gear is stabilized. Thus, there arises a problem in that an abnormal load exceeding the relative-value range is undesirably detected even if the load is in a normal state.